A diagnostic ultrasonic imaging system for medical use forms images of tissues of a human body by electrically exciting an acoustic transducer element or an array of elements to generate a controlled beam of acoustic waves with short duration emitted acoustic pulses that are caused to travel into the body. Echoes from the tissues are received by the acoustic transducer element or elements and are converted into electrical signals. The electrical signals are amplified and used to form a cross sectional image of the tissues. Echographic examination by transmitting and receiving acoustic wave energy is also used outside of the medical field for interrogation into other mediums of interest.
A conventional ultrasonic transducer is formed of a piezoelectric material, such as lead zirconium titanate (PZT), that has undergone a poling process to become macroscopically piezoelectric. The poling process is one in which the piezoelectric material is raised to an elevated temperature and subjected to a strong electric field to align dipoles of the material. The temperature must be greater than the Curie temperature of the material that marks the transition between ferromagnetism and paramagnetism. The Curie temperature is typically greater than 100.degree. Celsius.
The electrical field provided during the poling process aligns the microscopic polar regions of the piezoelectric material, i.e. the dipoles are aligned. Allowing the temperature to fall below the Curie temperature while maintaining the poling field fixes the dipoles in alignment. In this manner, the piezoelectric material remains macroscopically polarized, even after the poling field is removed.
Kawabe describes an ultrasonic transducer in U.S. Pat. No. 4,825,115. The transducer has an azimuthal direction and an elevation direction. As noted in the patent, the beam of ultrasonic pulses from a particular piezoelectric element has a fixed expanse in the elevation direction, with the fixed expanse being determined by the length of the piezoelectric element and the wavelength of the output ultrasonic pulses. The patent further notes that in order to vary the focal length of the ultrasonic transducer, the piezoelectric element may be divided into a matrix of smaller piezoelectric elements. However, in the same manner as the single-element transducer, the resulting matrix has a generally fixed focal length in the elevation direction. Furthermore, the increased number of elements requires a larger number of interconnections with more complex electronics. The scanning beam is typically controlled along the azimuth direction electronically, with the beam characteristics being fixed along the elevation axis.
U.S. Pat. No. 4,518,889 to 'T Hoen describes an ultrasonic transducer that is apodized to reduce side lobe levels by causing the level of response to vary as a function of the position on the transducer aperture. For example, the polarization of a piezoelectric body maybe controlled by locally polarizing regions of the transducer with different voltages or for different periods of time. A tailored polarization profile can be achieved by means of the apodized transducer, but again the result is a fixed focal length.
Ultrasonic imaging of a number of bodies having different depths may be performed by employing separate devices, with each device being tailored to a different, but fixed, elevation focal length. However, such an approach is not likely to be cost efficient. Moreover, the body of interest may be so large as to prevent high resolution imaging by a transducer having a fixed focal length along an elevation plane, so as to require a variable elevation aperture size.
Another concern in the design and operation of an ultrasonic device is suppressing lateral modes of vibration. Undesired lateral modes may arise from a number of sources. For example, fringe electrical fields may generate lateral modes during the transmission of acoustic waves. Additionally, bodies that are adjacent to a body of interest will reflect acoustic waves, even though the waves are focused at the body of interest. The lateral modes will adversely affect the ultrasonic imaging process.
What is needed is a device and method for transmitting and receiving acoustic waves such that the focal length of the device can be varied as desired by varying the effective aperture size along the elevation plane, as well as the azimuth plane. What is also needed is such a device and method that suppresses lateral modes of vibration.